VP's are major air pollutants. Authorities have taken steps to combat VP pollution by promulgating and enforcing regulations designed to effect major reductions in the quantities of those substances released to the surrounding environment. Current control statutes are mostly unrealistic. Attempted enforcement and compliance with these statutes is creating monumental difficulties.
The pollution control industry in its current form is of relatively recent origin and does not have an adequate inventory of cohesive systems or techniques with which to meet many of the new mandatory requirements. Indeed, it is not unusual for little or no improvement whatsoever to result from expanded application of known pollution control equipment using traditionally acceptable chemical treating agents. Sometimes, the result is a worsening of the situation, the only constant to date being an unremitting economic loss hidden in inflated costs of American-produced goods.
Scrubbing systems are perceived as perhaps the best currently available for VP control. A typical effluent scrubbing system today uses essential oils to provide a so-called vapor phase reaction. While this has correctly never been viewed by competent persons as an appropriate primary control step, additions of aromatics and essential oils are being promoted and actually used as controls for VP emissions. This only modifies human olfactory responses to some VP's. It does not rid the environment of VP's or otherwise remediate the damage caused by the release of VP's to the environment.
There are difficulties, too, in applying other techniques used in the past for the treatment of VP's--reaction of the offending VP with hydrogen peroxide, chlorine, or chlorine dioxide being typical examples. These materials are limited in application and can create as many or more problems than they can solve. For example, adding hydrogen peroxide to styrene resin vapors, a ubiquitous and troublesome VP, might provide realistic reductions in air effluents but not without creating significant problems such as increased explosion and fire hazards.
Compounding the problem is the enormous number of chemical species that are involved.
The plastic industry generates considerable air pollution in the form of resin vapors and solvents such as acetone, methyl ethyl ketone, and others.
It is common for chlorine to be proposed as a control for these and other VP's. Typically, chlorine is at best no more than marginally effective. In some cases, it actually exacerbates the problem by producing partially chlorinated effluents which may have a worse impact on the environment than the untreated products.
Chlorine is also widely and somewhat effectively employed for the removal of biological volatiles such as those generated by rendering, food processing, sewage, sludge, and such. Chlorine, however, is not very effective against most VP's including styrene and similar resins, phenols, terpenes, sesquiterpenes, petroleum, asphalt tars, and many other aliphatic and cyclic hydrocarbons.
Wood products evolve VP's during kiln and other forms of drying. The effluent can contain a wide variety of terpene- and terpenoid-type VP's.
Typical are southeastern pine shavings or chips. These wood products are used for making particle board. In this particular process, wood chips are dried in a rotary kiln. Once the chips have been conditioned by drying, they are subjected to processes which convert them into sheet board stock. This procedure is proprietary but involves the addition of adhesives and fillers to the chips and the subsequent formation of the treated chips into the completed products.
During chip drying, water is driven from the chips; and volatiles are more-or-less simultaneously driven off the chips with the water. Analysis of the emissions shows that alpha- and beta-pinenes, l-limonene, and camphene are principal components. All of these compounds are terpenes, and all are VP's.
Different tests using proven designs such as countercurrent and reverse spray or jet scrubbers charged with aqueous suspensions of more-or-less conventional scrubbing solutions containing chlorine, chlorine dioxide, potassium and sodium hydroxides, potassium permanganate, hydrogen peroxide, ozone, and the like have demonstrated little if any reductions in VP process effluents generated in the forementioned and other types of wood product production.
VP's originating from biological processes and biowastes have become increasingly more difficult to treat as volumes have grown to enormous proportions in some locales. Sewage; sludge; municipal solid and sanitary wastes; and food, fish, meat, agricultural, compost, and related processing have become very intensive, creating new problems characterized by increased volumes, complex blends and mixtures, and handling restrictions.
Traditional control systems in many cases are only marginally effective against these pollution sources. In other cases, useful control techniques and systems are simply not available.
While scrubbing is perhaps the technique most often used for removing pollutants from gases, the use of zeolites for this purpose has also been proposed. Zeolites are hydrated aluminum and calcium or sodium silicates--characteristically porous solids which require "activation" to become effective. Referred to as molecular sieves, zeolites are to different degrees effective in a variety of applications involving the treatment of volatile organic compounds.
Thus, U.S. Pat. No. 2,921,970 proposes the use of a single, contiguous, gravitating bed of zeolites for sorbing and separating into different zones hydrocarbons of differing weight. The proposed system attempts to overcome the cumbersomeness of dealing with a bed of solids under practical conditions with a polyvalent, cationic, silicate complex.
U.S. Pat. No. 2,944,033 discloses the use of insoluble exchange resins made from non-halogenated copolymers of styrene and maleic anhydride crosslinked with diamines in oxidizing various chemical compositions.
U.S. Pat. Nos. 2,988,502; 2,952,630; and 2,950,336 are concerned with similar products and systems for capturing, separating, and treating aromatic, paraffinic, and other hydrocarbons utilizing a number of different zeolites. Naturally occurring zeolites such as chabazite and analcite and synthetic zeolites are proposed.
U.S. Pat. No. 3,676,330 discloses an improved composite of a zeolite and a coating material for use as a catalyst. U.S. Pat. No. 3,719,026 proposes the use of an aluminum deficient zeolite in extracting non-polar substances from mixtures of polar and non-polar substances.
U.S. Pat. No. 4,309,281 discloses a calcined zeolite for separating non-aromatic compounds from aromatic compounds. U.S. Pat. No. 4,425,143 proposes yet another zeolite-based technique for removing impurities from gases. U.S. Pat. No. 4,529,416 discloses the use of sodium mordenite pellets in adsorption columns.
U.S. Pat. No. 4,564,604 is concerned with a catalyst comprised of protonized zeolite. This catalyst is claimed to be effective against oxides of nitrogen if they are washed with ammonia before passing across the catalyst. U.S. Pat. No. 4,544,378 proposes that faujasite-containing compositions be used for this same purpose.
Most of these patented applications require sorption and desorption cycles, activation of the zeolite, and complex zoned beds. They present difficulties with plugging and in keeping the bed at the more-or-less precise temperature needed for efficient operation. As a result, none of these patented techniques are practical for effluent control. Also, capital, operational, and maintenance expenses are prohibitive. Moreover, the efficiencies required for the applications to be practical in the real world are, with certain notable exceptions, all but impossible to accomplish within economic limits.
Still other techniques for separating out VP's are disclosed in the patent literature.
U.S. Pat. No. 3,403,498 discloses a method for removing saponifiable foreign substances from a gas stream by directing the effluent through a labyrinth. It is stated that saponifiable substances will collect by impingement on the labyrinth structure. The collected materials are then removed from the labyrinth with a caustic soda solution.
Proposed in U.S. Pat. No. 4,528,001 is the recovery of volatile organic matter from gases and gas mixtures via gas/liquid contact with an aqueous system. That system includes a hemiether or hemiester of an polyalkylenepolyol. A two-step process designed to recover resources and prevent pollution is employed.
U.S. Pat. No. 4,426,210 discloses a process for scrubbing odorous effluents with a solution of polyethylene glycol sorbitan laurate or an ester of sorbitan with palmitic acid and water. This process is said to be capable of effectively eliminating many volatile organic compounds from process effluents.
While the foregoing patents address the problem of cleaning effluent streams contaminated with VP'S, few are of any practical use in pollution control. They are limited by capital, operational, and maintenance requirements and exhibit impracticalities such as low efficiency attributable to bleed through, clogging, and surface plugging.
U.S. Pat. Nos. 2,492,085 and 2,906,668 disclose compositions containing aluminum chlorohydrate, zirconyl, aluminum halohydroxy complexes. The patented compositions are intended to be used as personal body deodorants and antiperspirants (the use of these compositions and complexes for VP control has not been proposed).
Another process that makes use of chlorinated compounds and which can also be employed in removing pollutants from air streams is disclosed in U.S. Pat. No. 4,844,721. That patented process, however, requires a scrubbing medium which contains a drying or semidrying oil with a high iodine value. The requirement for such oils in high concentrations can make the scrubbing medium too expensive for many VP control applications. Also, this patent is concerned with the recovery and regeneration of scrubbing media and not the devolatization of pollutants.
While ultraviolet radiation and air may be of some benefit in separating collected pollutants from treating media, they also accelerate polymerization and oxidative degradation of the treating oils used for scrubbing out pollutants. This reduces the efficiency of the medium and forms resinous masses and coatings which require frequent and difficult treatment. This can also make it more difficult to separate and handle recovered pollutants.
In short, the present day pollution problems attributable to VP's are pervasive and of enormous magnitude. Available techniques for combatting these problems are only marginally effective at best; and there are associated with available VP control techniques such other drawbacks as high capital, operating, and maintenance costs and the generation of products which are themselves pollutants.